Micro-viscometers of this type are used, inter alia, in affinity viscometry for determining the glucose content of an analyte, for example of blood. For this purpose, a separating membrane that allows a selective exchange of molecules is arranged between analyte and a measuring fluid, whereby the viscosity of the measuring fluid is directly related to the glucose content of the analyte.
The operating principle of the measuring fluid is based, for example, on the exchange of dextran molecules occupying the sugar-binding sites of a lectin molecule with glucose molecules that diffuse through the separating membrane. Other versions use a polymer solution containing boric acid functional groups as the measuring fluid, as described in WO 2010/0123521.
Micro-viscometers that are particularly suitable for use in affinity viscometry are known in the widest range of embodiments. Basically these embodiments can measure the viscosity by measuring the flow resistance of a moved measuring fluid in a capillary tube (e.g. DE 100 10 539 A1) or by a measuring body moved in the measuring fluid. For miniaturization, micro-electromechanical actuators are used, for example in the form of flexible tabs (cantilevers), which are moved in the measuring fluid and the dynamic response of these cantilevers is measured for a defined drive force. Measuring devices of this type based on a micro-viscometer therefore necessarily contain a drive and a system for measuring the cantilever deflection. For a known geometry and assuming a laminar fluid flow, the viscosity can be determined from the curve of the speed at which such an actuator moves through the measuring fluid for a defined drive force.
U.S. Pat. No. 5,955,659 describes an electrostatically driven cantilever for determining fluid properties. FIG. 1 shows a schematic diagram of this micro-viscometer according to the prior art. Under an electrode 106 of an actuator in the form of an elastically deformable cantilever 105 is located a counter electrode 104 in the form of a ground electrode on a silicon substrate 101 that has an electrically insulating coating 103. Switching on a voltage Vin between electrode 106 and counter electrode 104 produces a force of attraction between the electrode 106 and the counter electrode 104 that deforms the cantilever 105. The cantilever is regarded here as a spring/mass system, the movement of which is damped by the viscosity of the fluid to be measured. A simple embodiment of a measuring device comprises a contact which is closed as a result of a predetermined deflection of the cantilever. The time from switching on a drive voltage between cantilever and the ground electrode in the measuring chamber until the contact closes is hence used as a measure for the viscosity. If the drive voltage is then switched off, the elastic cantilever returns to its initial position, and the measurement process can be started again. In a conductive fluid, e.g. in an isotonic saline solution, this form of electrostatic drive cannot be used because the electrodes become polarized within fractions of microseconds, which reduces or eliminates the E-field in the fluid that is required for the drive, or additionally at higher voltages, electrolysis starts, releasing hydrogen. Documents DE 100 27 684 and U.S. Pat. No. 5,955,659 do not discuss these problems.
DE 100 27 684 A1 describes, according to one embodiment of the actuator, an electrical conductor to which a radio-frequency (RF) alternating current is applied and which interacts with another conductor, wherein at least one of these conductors is elastically deformable. A measuring device, which is not described in greater detail, performs a capacitance measurement or an impedance measurement between the movable conductor and a fixed conductor. It recommends choosing the excitation frequency and the frequency used for the impedance measurement to be so high that the force acting on the actuator and the impedance measurement are largely independent of the electrical conductivity of the measuring fluid.
In fact it is advantageous to use for driving the cantilever or a differently designed, elastically movable actuator, an alternating voltage having a frequency that is chosen to be so high that the effects of ion movement and electrode polarization in the measuring fluid on the force moving the actuator are small. The conductance and the frequency dependency of the permittivity of the measuring fluid must be taken into account both for the drive and for the measuring system. Using a DC voltage or a low-frequency voltage is not only unsuitable for the actuator drive in a conductive fluid but also, owing to an ion current and polarization effects on the electrodes, will result in an incorrect capacitive distance measurement, for example.